In a process for producing butadiene from n-butane employing two dehydrogenation steps, the product of the first dehydrogenation step is essentially a mixture of n-butane and butylenes. For the following dehydrogenation step, it is desirable to separate this mixture so that only the butylenes are introduced into the second dehydrogenation zone. This separation step can be carried out by extractively distilling the n-butane/butylene mixture using as extractive solvent a mixture of a sulfolane and acetone. In this extractive distillation, some acetone leaves the distillation column overhead with the n-butane product that also leaves the distillation column overhead. For further processing or recycling of the ingredients, it would be desirable to have a process available by which this mixture of n-butane and acetone can be effectively separated. It is not feasible to separate the n-butane and acetone by ordinary distillation.